Uninterruptible power supply (UPS) systems for homes and facilities have been deployed for many years and typically consist of a 48 Vdc lead-acid battery pack connected to an Inverter/charger which is connected to the power grid and to the home or facility. The Inverter/charger is adapted to monitor the state of charge of the battery pack and to recharge the battery pack when required preferably when power consumption is low and/or when the energy companies charge less for electricity such as overnight. The Inverter/charger also provides the functions of: a) transferring the AC power from the grid directly to the home or facility when power consumption is normal; b) injecting power from the battery pack to the home or facility when power consumption exceeds the capacity of the power grid or for peak shaving to ensure that power consumption of the home or facility does not exceed a threshold where the electricity becomes more expensive; and c) supplying back-up power to the home or facility exclusively from the battery pack during the power grid outages.
The Inverter/charger charges the lead-acid battery pack by taking the input AC power from the grid, converting it to a DC power and directing the DC current to the battery pack. Conversely, the Inverter/charger supplies power from the lead-acid battery pack by taking the DC power from the battery pack, converting it to AC power and directing the AC current to the home or facility.
Standard inverter/chargers for lead-acid batteries use a simple algorithm to recharge the batteries. As shown in FIG. 1, lead acid batteries are typically charged in three stages: 1) a constant-current charge; 2) a constant voltage charge or topping charge; and 3) a float charge. The constant-current charge applies the bulk of the charge and takes up roughly half of the required charge time; the topping charge continues at a lower charge current to maintain the voltage of the battery and provides saturation, and the float charge compensates for the loss caused by self-discharge. The switch from Stage 1 to 2 occurs seamlessly and happens when the battery reaches the set voltage limit. The current begins to drop as the battery starts to saturate, and full charge is reached when the current decreases to the three percent level of the rated current.
Lithium Metal Polymer (LMP) batteries and to a lesser extent, lithium-ion batteries, require a more complex algorithm for recharging than lead-acid batteries due to their specific chemistry and to the need of monitoring and controlling their temperature during charge, discharge and floating and the specific need to monitor the voltage of each individual cells of the batteries. For these reasons, it is not possible to simply replace a lead-acid battery pack with a lithium battery pack in an existing Uninterruptible power supply (UPS) system for homes and facilities. The entire system including the inverter/charger must be replaced.
Lithium batteries have higher energy density that lead-acid batteries, require less maintenance and last longer than lead-acid batteries and are therefore ideally suited for Uninterruptible power supply (UPS) system for homes and facilities. However, the initial cost of replacing the entire UPS system instead of just the battery pack has refrained home and business owners from switching to Lithium batteries even if it represents saving in the mid to long term range (3-5 years).
Thus, there is a need for a circuit device adapted for linking a lithium based battery pack to a standard inverter/charger for lead-acid batteries and therefore allow the replacement of a lead-acid battery pack with a lithium based battery pack.